Electron tube



Dec. 13, 1932. B, F, MlEssNER 1,890,911

ELECTRON TUBE FiledvMarch l, 1930 2 Sheets-Sheet 1 Dec. 13, 1932. B, F,- MlEssNER' 1,890,911

ELECTRON TUBE @imag Z 'Z/ C 7 2/ Patented Dec. 13, 1932 UN'rED STATES BENJAMIN "n". MIESSNEJLKOF SHORT HILLS, NEW JERSEY, ASSIGNOR y'TO MIESSER` Parri-:Nrl oF-Ficr;

INVENTONS INC., AF SHORT HILLS, NEW JERSEY, A- CORPORATION OF NEW JERSEY ELECTRON TUBE Application filed March 1,

This invention relates to space discharge devices of thermionic type and particularly to the type in which the cathode is a unipotential cathode. This structure usuallyA embodies a conductive thimble indirectly heated, chiefly by radiation and conduction from a. heater element energized by alternating current.

The primary object of my invention is to minimize or substantially eliminate hum introduced into the output circuit of the tube by fluctuating temperatures, voltages or ourren'ts within the heater element when the same is energized by alternating current.

Other objects of my invention are the production ofrigid and inexpensive structures in which various spacings of the elements will be maintained constant throughout the life of the tube as will be more particularly n pointed out as the description thereof is developed in the following specification.

ll have discovered that in this type of tube, having` an indirectly heated cathode, one of the chief sources of hum in the output of the tube is due to the magnetic field of the heater current. Other features contributing to hum are the leakage of heater current through theV insulating material into the cathode, and voltage effects across segments of the heater element itself, as well as voltage effects between the heater element and the cathode thimble. Tubes of present-day manufacture utilize a straight or helically coiled wire extending through the cathode thimble with or without an intervening insulator to prevent contact with the cathode thimble, or a hair-pin type heater with straight parallel legs eX- tending through a cylindrical insulator having two holes longitudinally throughout and arallel to its aXis.

The heater wire in this latter type is round and the separation between the two sides or legs of the hair-pin is about 1/3 the diameter of the cylindrical insulator, which fits snugly into the cathode thimble.

The separation of the heater legs in such a type of cathode produces a considerable magnetic field, incompletely neutralized, which reaches out into the electron stream near-thecathode surface and there .causes a 1930. semi No. 432,286.

change Vof direction in the flow of the electrons tothe plate periodically with the rise f electron from its source of origin to the plate This variation 1n or anode of the tube. length of path over which `the electrons travel causes the hum component present in the curret flowing inthe output circuit of such a tu e. -f

An analysis of this'structure shows that in the plane bisecting both heater'legs the distance between a leg and the nearest ycathode wall portion is about equal to the separation between the heater legs so that near this plane there is; considerable unneutralized field at the cathode surface.

I have developed al combination of two features which I have included in the structures to be described hereinafter for minimizing the fluctuations in the `magnetic field atvthe surface of the cathode, iirst by maintaining the centers of the legs as closetogether as possible, thereby producing more nearly perfect neutralization between the `magnetic fields thereof and, second, by locating .the emissive surface of the cathode'thimble at such distances from the 'heater element that substanthe two legs of the heater-element is obtained Y throughout f the cathode thimble surface. This application being a continuation in part of my application ySerial Number 431,121 filed February 25, 1930.

Referring to the drawings, in which like reference characters have been used throughout insofaraspossible, Fig. l diagrammatically illustrates a compl-ete electron tube vof my improved form.

Fig. 2 shows the tube of Fig. 15 partly in section to disclose the relationship of the cathode connection and the connections to the heater element in relationship to the grid and plate leads.

Fig. 3 diagrammatically illustrates acrosssection vvof the tube of Fig.k l on the line 3 3.

the socket pins 3 Fig. 4 is a cross section of a cathode of my improved form;

Fig. 5 is a cross section of a modified form of my invention;

Fig. 6 is a modification of the type of cathode shown in Fig. 5;

Figs. 7 and 8 show the heater element of my improved cathodes as shown in Fig. 6 in the process of formation.

Referring to Figs. 1 and 2, 1 designates a base having five prongs designated 2 and 6 respectively, such as employed in the standard base of heater type tubes of present-day manufacture. 'Ihisbase carries a glass envelope 7 preferably evacuated and within which there is a stem 8. Upon this stem 8 there is rigidly mounted an anode 9 either cylindrical or oblong in cross-section surrounding a grid 10 and a cathode 11. The cathode is provided witlrtwo heater connections 12 and 12 securely mounted within the stem 8 and in continuation of lead-in wires 13 and 13 which in turn are connected to and l respectively. The cathode thimble is provided with a connection 14 sealed through the stem 8 and in continuation of a lead-in wire 15 which in turn is connected to prong 6.

In these figures it will be noted that the cathode lead-in 14tis disposed between the lead-ins 12 and 12 to the heater element on one side and lead-in 10 to the grid on the other side. This feature is to eliminate insofar as possible any induction in the lead-in Wire to the grid of the tube.` This is accomplished since the cathode lead-in is usually grounded and thereby acts as a shield between these elements.

The cathode thimble itself may either be cylindrical, or nearly elliptical as shown in Figs. 46, details of the description of which follow hereinafter.

In such a cathode structure the heater element is so arranged within the cathode thimble that the bend of the heater element does not protrude from the upper end of the cathode thimble, and I have found that a safe distance below the end of the cathode thimble for the bend in a hair-pin is approximately .06 inches, as an example of one of the tubes I have constructed in accordance with the features of my invention. The heater wires themselves at the bottom of the thimble should be as short as possible and connected to rugged supporting members of large heat carrying capacity so that substantially no emission occurs from the heater element terminals protruding from the cathode thimble as shown in detail in Fig. 1.

In Fig. 4 I have shown a cross section of one form of my improved heater element in which numeral 16 refers to a cylindrical insulating element of isolantil'e, fused quartz, magnesium oxide, or like insulating materials having adequate insulating properties at the required operating temperatures and having two cylindrical holes 17 and 18 through which the legs 19 and Q0 of the heater element are passed. The insulating member 16 is of a type commonly used in slow heater cathode types of tubes available on the market at the present day, but in which I have spaced holes 17 and 18 at a distance of approximately 0.005 inches from the center C instead of at a distance of 0.016 inches as now practiced. Surrounding the insulating member 16, I have provided a cathode thimble 21 of nick-el, approximately 0.001 inches thicli and of a diameter of 0.1 inches.

I am aware that heretofore it has been sug gested to space the two heater legs or, speaking more generally, two sections of the con` ductor carrying current of opposite flow direction at close proximity in order to keep the resultant magnetic field produced by these legs within low intensity values. I-Iowever, such prior constructions have overlooked the fact that the deciding factor in reducin g the field intensity at the outer surface of the surrounding thimble is not to be found in the absolute value of the distance between the two conductors of opposite current sign, but, rather, in the ratio of this distance to the diameter of the thimble. By selecting this ratio at a small value of 1 to 10 or less, as in the design of Fig. 4E, the field intensity in any element on the surface of the cathode th'mble is substantially completely neutralized due to the practical equal distance of any points in the emitting surface from the currents centers of the two legs.

In comparing the electrical constants of a cathode of this structure with these commen cially available at the present time, I have determined that the fluctuating components present in the plate circuit of the commercial. tubes is due to small portions of electron emissive material distributed about the surface of the cathode thimble 21 being at too widely differing distances from the two heater legs as pointed out in detail in my copending application hereinbefore referred to, while in the structure shown in Fig'. si the Ydimensions are so coordinated that the fields produced by the individual legs 19 and 20 of the hairpin heater at any point are substantially of the same opposing strength.

In my application hereinbefore referred to I pointed out in detail that emissive substances placed near the plane YY on the cathode were substantially equidistant from both legs of the heater element and therefore were not subjected to any fluctuating field. However, in the present case, the distance between the heater legs 19 and 20 disposed near or about the center C is also negligible compared to the distance between the center C and the points X and Z and, therefore, the entire cathode thimble will be substantially free from any fluctuating magnetic flux.

A modified form of my invention is shown in Fig. 5 wherein the heater element is made up of a ribbon the two legs of which are illustrated at 22 and 23 of Fig. 5. These legs are separated by a strip 24 of mica or like insulatof only 0.002 inches. The interior of the cathode is occupied `by an insulating lmaterial body as shown which may consist of grooved half cylinders of isolantite or powdered magnesia or like composition to hold the heater elements securely in place.

A cathode of this type is preferable to that shown in Fig. l since the centers of the heater legs 22 and 23 may be maintained so close together that the resultant field extending in any direction from the two legs 22 and QSisV very small. Therefore, even with spacing of the surface portions of the cathode thimble at comparatively short distances away from the heater element a very efficient and quiet cathode results.

As an example, I have inade a cathode of this modification in which the mica element 2d is 0.002 inches thick, the ribbon heater is .001 inches thick and .030 inches wide with a cylindrical cathode thimble being 0.001 inches thick and having a diameter ofv .050 inches within an electrode structure of the 227 type. rlfhis tube is substantially hum-free as regards hum developed from cathode energization by alternating current.

From the dimensions given for this cathode it will be noted that, though the diameter of the thimble is only one half of what it was in the example given above in connection with Fig. e, the critical ratio as determined by the distance between the centers of the ribbons relative to the length of the thimble diameter has even been reduced to l to 25 as against l to l0 in the design according to Fig. 4 thusmaking the design of Fig. 5 a more desirable one from both an operative and commercial point of view.

A modification of this structure is shown iii Fig. 6 wherein the heater element is identical with that shown in Fig. 5 and where the distance from the points X and Z respectively to the heater legs is likewise substantially the same.

In this forni however, I have availed myself of the fact-s pointed out in my application hereinbefore referred to namely that near the plane Y--Y of the cathode there is complete neutralization directly up to the thimble from the heater legs at such distances ing material having a thickness for example In a more generic aspect, then, the principle underlying'this invention may be explained as follows. In conventional, in-

directly heated cathodes the heater element consists of an elongated conductor (filament or ribbon) whose portions of opposite current flow are ent upon themselves in close contiguity to each other. This heater element extends interiorly of said thimble usually coaxially therewith and it produces in virtue of the fluctuating electro-magnetic field linked with it more or less disturbing effects in different elements of the surface of the thimble on the electron stream emanating from this surface. Considering, then, two contiguous portions of opposite current flow in said heater element in view of the requirement of keeping those disturbing effects within desired low limits, it will be necessary to determine the ratior of the shortest distance between the current centers of any two such contiguous portions toV a distance. as traced by dropping a perpendicular from the midpoint of that shortestdistance ontoany surface element in the electron-emissive surface of the thimble in such a manner that said disturbances are kept within desired low limits. Obviously, the disturbing effects are the lower, the lower is said ratio. From my experiments I have found that this rat-io should be at most l to 10.

If, in addition, I prefer to make the distance determining the degre of proximity or contiguity between ytwo portions of opposite current flow of the heater element as small as compatible with a safe insulation value between such contiguous portions,`it will now be understood that this is a provision made in view of reducingthe thimble diameter as far as possible for a predetermined value of the above specified ratio. In other words, it `is solely this ratio which determines the operative behaviour of the cathode under. the objects of this invention, whereas the absolute value of said distance between contiguous portions is, primarily, a determining factor under commercial considerations and has no bearing on the disturbing effects of electromagnetic nature.

The forni of heater element preferably employed for the designs in Figs. 5 and 6 is shown in detail in Fig. 7 in which I have provided a strip ofmetal-ribbon of the size previously mentioned, designated as 4G bent at the point 47. Between the bent portions of this ribbon I inserta plate of mica 48 approximately 0.002 inches thick, which I have found to be sufficient thickness to maintain substantiallyperfect insulation between the parallel sections of the heater element. This strip is indented as shown at 49, so that the bend of the ribbon may be inserted therein and its facing sides be maintained in iixed relation when contacting the outer sides of the strip under pressure from the holding elements and 5l, which likewise may be strips of mica as shown in Fig. 5, or grooved pieces of insulating material as shown in Fig. 6. Each of these holding elements may be notched as shown at 52 and 53 so that the ends of the ribbon 46 may be bent through the notches 52 and 53, thus securing the insulating material in positive alignment for mounting within the cathode structure. As previously mentioned the cathode thimble it self may be used to then clamp the structure securely together. However the preliminary use of a binding wire wound therearound or other mechanical arrangements may be employed without departing from the spirit of the invention as defined in the appended claims.

While I have shown three individual str-uctures and have given dimensions for particular examples of tubes embodying my invention, I am not to be limited thereby, since a tube in which the cathode thimble and the electron emissive material thereon is placed at points of substantially neutralized magnetic field, with respect to the heater element, comes within the scope of my invention as illustrated in the foregoing figures and delined in the appended claims.

Having thus described my invention, what I claim is:

1. A cathode for electron emission including an elongated thimble of electron-emissive outer surface and a bent-upon-itself electrie heater element extending interiorly of said thimble substantially co-axially therewith, the ratio of a shortest distance betweencurrent centers of two contiguous portions of opposite current flow of said heater element to a distance as traced by dropping a perpendicular. from the midpoint of said shortest distance onto a surface element in said electron-emissive thimble having for each said element a numerical Value of at mostV l divided by 10. 1

2. A cathode for electron emission including an elongated thimble of oblong crosssectional outer shape and of electron-emissive outer surface, and an inverted U-shaped electric heater element extending interiorly of said thimble substantially co-axially therewith, the legs of said U-shaped heater element being bisected by a plane which extends parallel to a plane passing through the minor axis of said oblong cross-sectional shape, the legs of said heater being closely spaced to each other and each point on said thimble being substantially equidistant from the nearest points along the current centers respectively of said two heater legs.

3. A cathode for electron emission including an elongated thimble of oblong cross-sectional outer shape and of electron-einisslve outer surface, and an inverted U-shaped elec tric heater element extending interiorly of said thinible substantially coaxially therewith, the legs of said Ushaped heater element being bisected by a plane which extends parallel to a plane passing through the minor axis of said oblong-sectional shape, the shortest distance between the center lines of said legs being of a Value as low as compatible with a safe insulation between said legs and each point on said thimble being substantial ly equidistant from the nearest points along the current centers respectively of said two legs.

4l. The combination with a cathode thimble adapted to evaporate electrons from its outer surface, of an electric heater unit fitting in said thimble in electrically insulated relation thereto, said unit including a thin elongated sheet of insulating material having an indentation intermediately disposed at one of its narrow edges, an elongated ribbon-like conductor being bent, intermediately of its length, about the edge of said indentation so as to have its facing sides contacting the sides of said sheet of insulating material, and insulating means bridged between opposite points on the interior wall of said thiinble and securing said heater in position.

In testimony whereof, I aflix my signature.

BENJAMIN F. llIESSNER. 

